1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus. More particularly, the present invention relates to an image transferring and sheet conveying device including a belt configured to convey a sheet or recording medium via a nip between it and an image carrier to thereby transfer a toner image from the image carrier to the sheet.
2. Description of the Background Art
Generally, in an electrophotographic image forming apparatus, after the surface of a photoconductive drum or image carrier has been charged, an exposing device directly exposes the surface of the drum or laser optics or LED (Light Emitting Diode) optics optically scans it in accordance with image data, thereby forming a latent image. A developing unit develops the latent image with toner to thereby produce a corresponding toner image. The toner image is transferred from the drum to a sheet or recording medium and then fixed by a fixing unit.
An image forming apparatus of the type described usually includes an image transferring and sheet separating device including a belt and configured to transfer the toner image from the drum to the sheet, then separate the sheet from the drum, and then covey it to the fixing unit. The belt, which is endless and faces the drum, is formed of rubber or similar elastic material and passed over a plurality of rollers including a drive roller, a driven roller and a bias roller for image transfer. The belt conveys the sheet via a nip between it and the drum while causing the toner image to be electrostatically transferred from the drum to the sheet. After the image transfer, the belt separates the sheet from the drum and conveys it to the fixing unit. A cleaning blade, cleaning brush or similar cleaning member faces at least one of the plurality of rollers via the belt in order to remove toner, paper dust and other impurities deposited on the belt.
Today, to meet the increasing demand for higher image forming speed, it is necessary to increase the linear velocity of the belt. In this case, a bias for image transfer must be lowered in order to guarantee expected image transfer. However, a bias of, e.g., 1,000V produces ozone and oxide gases including NOx (nitrogen oxides) during image transfer; the lower the bias, the greater the amount of oxide gases. Polychloroprene rubber is contained in many of conventional belts as a major component from the processing, durability and cost standpoint. However, because polychloroprene rubber is not sufficiently resistant to oxide gases, the belt cracks as image formation is repeated with a high bias for image transfer, resulting in defective image transfer and therefore defective images. The belt should therefore be frequency replaced.
The belt critically lowers image quality when slackened at the position where it faces the drum. In light of this, it is a common practice to stretch the belt during operation, compared to an unloaded condition. However, when the belt is continuously stretched during high-speed movement, the belt slackens or cracks as image formation is repeated and critically lowers image quality. The belt therefore should be frequency replaced.
A filler, e.g., carbon black or similar carbon or silica, alumina, talc or similar inorganic compound is, in many cases contained in the belt in order to improve the mechanical and electric characteristics of the belt. Considering the dispersion of the filler, the grain size of the filler should preferably be as small as possible. However, if the grain size is excessively small, then the mechanical and electric characteristics cannot be improved as expected, so that the filler content of the belt should be increased. This reduces the flexibility of the belt and renders the electric characteristics of the belt irregular and thereby causes the belt to easily crack due to repeated image formation while rendering image transfer irregular as well.
If the grain size of the filler is excessively great, the filler cannot be uniformly dispersed in the belt and makes the mechanical and electric characteristics of the belt locally irregular. This also results in cracks and irregular image transfer.
Even though the belt may be provided with a persecuted composition, any change in the thickness or the circumferential length of the belt directly translates into a change in, among others, the durability of the belt, making the above composition useless in many cases. A new composition must therefore be studied by trial and error. This is particularly true when the thickness of the belt is changed.
Further, to meet the increasing demand for size reduction of the image forming apparatus, it is necessary to reduce the size of the image transferring and sheet conveying device as well. The size of this device may be reduced if, e.g., diameters of the rollers over which the belt is passed are reduced. However, if the roller diameter is reduced to, e.g., 20 mm or below, heavy stress acts on the belt due to curvature and noticeably reduces the life of the belt up to the appearance of cracks.
The protection of rubber from deterioration ascribable to ozone and other oxide gases has long been a target to tackle in the rubber industry. For example, it has been customary to add an anti-deterioration agent, which reacts with ozone and inactivates it, to rubber or to coat it on the surface of rubber. Typical of the anti-deterioration agent are an amine-ketone condensate, aromatic secondary amine or similar amine-containing agent, a monophenol derivative, polyphenol derivative, hydroquinone derivative or similar phenol-containing derivative, a sulfur-containing agent, and a phosphor-containing agent. However, the anti-deterioration agent practically looses the expected effect when fully reacted with ozone. This, coupled with the limited allowable content of such an agent, makes it impossible to sufficiently extend the life of the belt. Particularly, when the bias for image transfer is xe2x88x921,000 V or below, the life of the belt is short because of a great amount of ozone produced.
It has also bee customary to cope with the deterioration of rubber by adding, e.g., wax to rubber in order to physically isolate rubber from ozone by using the blooming of wax. However, although the wax-containing belt is highly resistant to ozone, the belt is apt to locally stretch or crack due to the low melting point of wax during repeated image formation, resulting in defective image transfer or defective sheet separation.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-34163, 9-90781, 11-30918, 11-352787 and 2001-122417.
It is an object of the present invention to provide an image transferring and sheet separating device insuring desirable image transfer and sheet separation over a long time without its belt being replaced, and an image forming apparatus including the same.
It is another object of the present invention to provide an image transferring and sheet separating device implementing high image forming speed and size reduction, and an image forming apparatus including the same.
An image transferring and sheet separating device of the present invention includes a belt facing an image carrier. The belt conveys a sheet via a nip between it and the image carrier to thereby transfer a toner image formed on the image carrier to the sheet by being applied with a bias and then separates the sheet from the sheet. The belt contains a filler having a grain size of 0.5% to 5% of the thickness of said belt. The belt is driven with circumferential length 1% to 7% greater than circumferential length in an unloaded condition.
An image forming apparatus including the above image transferring and sheet conveying device is also disclosed.